1. Field of the Invention
The present invention relates to a polyphenylene ether composition which not only has excellent flame retardancy and excellent impact resistance, but also is friendly to the environment. More particularly, the present invention is concerned with a polyphenylene ether composition comprising: 100 parts by weight of (A) a polyphenylene ether resin or a combination of a polyphenylene ether resin and a styrene resin; 1 to 40 parts by weight of (B) a flame retardant which is comprised mainly of at least one member selected from the group consisting of two specific types of condensed phosphoric esters; 0.3 to 10 parts by weight of (C) a hydrogenated block copolymer which is defined as a hydrogenation product of a block copolymer comprising a polymerized styrene block and a polymerized conjugated diene block, wherein at least one polymerized styrene block has a number average molecular weight of 15,000 or more; and 0.3 to 10 parts by weight of (D) a polyolefin resin, wherein the weight ratio of the component (C) to the component (D) is from 70/30 to 15/85. The polyphenylene ether composition of the present invention is friendly to the environment because the polyphenylene ether composition contains neither a halogen compound nor an antimony compound as a flame retardant. Also, the polyphenylene ether composition retains good heat resistance, mechanical characteristics and dielectric properties which are inherently possessed by a polyphenylene ether resin. Further, the polyphenylene ether composition of the present invention, in the injection molding thereof, exhibits excellent mold release characteristics and is substantially free from generation of a gas and from adhesion of the flame retardant to the mold, so that the molding cycle time of the polyphenylene ether composition can be shortened. Furthermore, the polyphenylene ether composition also exhibits excellent chemical resistance and excellent impact resistance.
2. Prior Art
Conventionally, as a method for imparting flame retardancy to a flammable synthetic resin, a method has been employed in which a halogen compound or a combination of a halogen compound and antimony trioxide is added to a flammable synthetic resin to obtain a flame retardant resin. However, this method is disadvantageous from the viewpoint of environmental hygiene since, when the flame retardant resin is burned, a poisonous gas is generated. Therefore, it has been desired to improve the above-mentioned method for imparting flame retardancy to a flammable synthetic resin.
With respect to each of a polyphenylene ether resin and a mixed resin comprising a polyphenylene ether resin and a styrene resin (hereinafter, this mixed resin is frequently referred to as a “PPE/styrene resin”), flame retardancy can be imparted to the resin by incorporating thereinto an organic phosphoric ester instead of a halogen compound and antimony trioxide. Examples of organic phosphoric esters include a monophosphoric ester, such as triphenyl phosphate, cresyldiphenyl phosphate or tricresyl phosphate, and a condensed phosphoric ester which is produced using as a raw material a polyhydric phenol (especially a dihydric phenol, such as resorcinol or bisphenol A). It is generally said that the above-mentioned organic phosphoric esters do not adversely affect the environmental hygiene as compared to the case of the halogen compounds. It is also said that a resin composition containing a condensed phosphoric ester is better than a resin composition containing a monophosphoric ester in that the resin composition containing a condensed phosphoric ester, in the injection molding thereof, has excellent heat resistance and is relatively free from generation of a gas and from adhesion of the flame retardant contained therein to the mold, so that there is a growing demand for a resin composition containing a condensed phosphoric ester.
However, the above-mentioned resin composition containing a condensed phosphoric ester has disadvantages. For example, although the resin composition containing a condensed phosphoric ester has improved properties related to stiffness, such as tensile strength, flexural strength and flexural modulus, as compared to those of the resin composition containing a monophosphoric ester, the resin composition containing a condensed phosphoric ester does not retain a good impact resistance which is inherently possessed by the polyphenylene resin, so that the resin composition is likely to suffer cracking. Further, with respect to the improvement of flame retardancy, a condensed phosphoric ester is less effective than a monophosphoric ester. Therefore, it is necessary to use a condensed phosphoric ester in a large amount as compared to the case of a monophosphoric ester. However, when the resin composition contains a large amount of a condensed phosphoric ester, the impact resistance of the resin composition becomes further lowered. Therefore, it is necessary to prevent the impact resistance of the resin composition from lowering. For this purpose, for example, a large amount of an elastomer is incorporated into the resin composition. As an elastomer used for improving the impact resistance of a PPE/styrene resin, a hydrogenated block copolymer having excellent heat stability is generally used. However, for satisfactorily improving the impact resistance of the PPE/styrene resin, it is necessary to use the hydrogenated block copolymer in a large amount. However, the use of a large amount of the hydrogenated block copolymer is disadvantageous not only in that the flame retardancy of the PPE/styrene resin is lowered, but also in that the use of the hydrogenated block copolymer which is expensive is uneconomical.
As a conventional method for further improving the impact resistance of a PPE/styrene resin composition (i.e., composition comprising not only a polyphenylene ether resin and a styrene resin, but also another component, such as a condensed phosphoric ester), there can be mentioned methods using a hydrogenated block copolymer in combination with a polyolefin resin, which are disclosed, for example, in U.S. Pat. Nos. 4,145,377, 4,166,055 and 4,383,082, Unexamined Japanese Patent Application Laid-Open Specification No. Sho 56-51356, Unexamined Japanese Patent Application Laid-Open Specification No. Sho 62-179561, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 2-169665 (corresponding to EP 0 362 660), Unexamined Japanese Patent Application Laid-Open Specification No. Hei 4-7357 (corresponding to U.S. Pat. No. 5,296,540), Unexamined Japanese Patent Application Laid-Open Specification No. Hei 4-279697 (corresponding to U.S. Pat. No. 5,296,540) and Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-192561. However, the conventional techniques disclosed in the above-mentioned patent documents are not concerned with a flame retardant resin composition containing a condensed phosphoric ester as a flame retardant, that is, use of these conventional techniques is not intended with respect to the resin compositions having flame retardancy. Therefore, the objects of the inventions of the above-mentioned patent documents are different from that of the present invention, and the amounts of a hydrogenated block copolymer and a polyolefin resin used in the above-mentioned patent documents are large as compared to those used in the present invention. Generally, the more the amounts of a hydrogenated block copolymer and a polyolefin resin, the more improved the impact resistance of the resin composition, but the more lowered the flame retardancy of the resin composition. For improving the impact resistance of a flame retardant resin composition containing a condensed phosphoric ester as a flame retardant (wherein the resin composition has poor impact resistance), Unexamined Japanese Patent Application Laid-Open Specification No. Hei 09-151315 attempts to incorporate a grafted copolymer into the resin composition, wherein the grafted copolymer is obtained by grafting a hybrid rubber onto a polyorganosiloxane as a base polymer. In this case, the impact resistance of the resin composition can be improved without lowering the flame retardancy of the resin composition. However, the grafted copolymer used for improving the impact resistance of the resin composition is expensive and, hence, the use of such a grafted copolymer is economically disadvantageous.
A PPE/styrene resin composition containing a phosphoric ester having high acid value is susceptible to absorption of water and hydrolysis. By the hydrolysis of the PPE/styrene resin composition, the dielectric properties (i.e., dielectric constant and dielectric loss tangent) of the PPE/styrene resin composition are lowered.
The molding of a PPE/styrene resin composition (especially a flame retardant PPE/styrene resin composition containing a flame retardant) is frequently conducted by injection molding. In this case, there is a problem in that a shaped article obtained by the injection molding exhibits poor releasability from the mold. It has been desired to improve the releasability of the shaped article from the mold in the injection molding.